I don't think there's any written requirement (we're all in some sort of fraternal order because we're cyclists after all), but it's just nice to ask "Mind if I draft awhile?" Then open up, "Hi, my name is...". Drafting is a fundamental part of cycling.
However, drafting, if effective, does involve someone's wheel within inches of yours. I wouldn't worry about it, if his front wheel touches your rear wheel, he's going down, not you--you won't even notice
